Anti-virus researchers have turned up three Trojan horse variants that are the most successful attempts yet to steal money from smartphone users.
The variants on the Viver Trojan (short for Trojan-SMS.SymbOS.Viver) send text messages to premium-rate numbers in Russia, a tactic used by some previous malware. But Viver is more sophisticated, according to Kaspersky Lab, which discovered all three Trojan variants last week.
Viver was uploaded to a popular file-sharing site and was downloaded by hundreds of users before it was removed, according to Kaspersky senior virus analyst Aleks Gostev.
The first pieces of malware to try the premium-number tactic were RedBrowser and Wesber, but those were written in Java and required user interaction for each message sent.
Because the earlier programs made use of Russian premium-rate numbers and didn't include a country code, they were only able to function properly from within Russia.
The Viver variants remove those limitations. To begin with, they are written for the Symbian platform - specifically Nokia's S60 version of the operating system, second edition and earlier versions, according to antivirus firm F-Secure.
"Viver is coded to run on phones with Symbian, making it the first Trojan of this type for smartphones," said Kaspersky's Gostev in a research note.
The new Trojans still text to a Russian number but use correct international dialling codes, and thus can work from any country, Gostev said. They don't require any user interaction, but simply begin sending texts as soon as they're installed.
Each text costs the user 177 roubles, or about £3.50.
Gostev said Kaspersky discovered the Trojans on a popular file-sharing site for mobile users, presenting itself as a photo editor, set of video codecs or other utility.
He said one of the Viver variants was downloaded by around 200 people in less than 24 hours, before the site administrator removed it.
Security experts said the Trojan represents a worrying trend.
"Prior to 2003 there was little for-profit malware on the PC platform, and now almost all malware is written for one or other profit motivation," said F-Secure researcher Jarno Niemela on the company's blog. "It is very likely that more for-profit malware will also appear on mobile platforms."
Kaspersky's Gostev said for-profit mobile malware already seems to be proliferating quickly. "This month alone we've logged three similar incidents," he wrote. "We can only guess how many more of these Trojans are out there, but one thing is for sure - if there's money to be made, virus writers won't be slow to take up the opportunity."